With a scanning transmission electron microscope (STEM), the smaller a diameter of an electron beam (a probe) scanning over a specimen is, the higher resolution as obtained is. A probe diameter is limited owing to the aberration of an electron-optical system; however, an apparatus with an aberration corrector mounted therein has lately been developed.
In general, an aberration corrector configures a plurality of multipole lenses, and a plurality of round electromagnetic lenses. Aberrations are measured by use of any method at the time when the aberration corrector is activated, and the respective lenses are provided with feedback in such away as to reduce the respective aberrations. For example, Patent Document 1 discloses a method for Fourier analysis of an autocorrelation function from segmented areas of a Ronchigram, as a method for measuring aberrations. Further, for example, Patent Document 2 discloses a method for measuring aberrations.